A reaction injection molding (R.I.M.) process is one in which two or more liquid resin components are rapidly mixed and injected into a mold where they react and cure to form a finished solid article. In a typical reaction injection molding process, for example, a polyol liquid is reacted with an isocyanate liquid to form a polyurethane. Such processes are sometimes alternatively referred to as liquid injection molding (L.I.M.) processes, or liquid reaction molding (L.R.M.) processes.
Such processes are often used to make relatively large articles with relatively inexpensive machines. For example, R.I.M. processes are used in making automobile parts and in the manufacture of shoe soles.
The resin components utilized are generally relatively viscous materials, sometimes having viscosities in a range from 10 to 1000 centipoises, for example. Such resins usually react quickly, some having a "gel" time as short as a few seconds. Mixing ratios by volume are normally in a range from about 3:1 to 1:1 and flow rates by weight are usually in a range from about 0.5 to 5 kilograms/second (i.e., about 1 to 10 pounds per second), although machines having larger flow rates have been available to the art. A typical operating time cycle might include an injection time of 2-5 seconds and a reaction and cure time of about 60 seconds.
Effective mixing of viscous resins is often achieved by using impingement mixers in which the fluid streams are injected at high velocities into a mixing chamber in a manner such that they impinge upon each other, the resultant mixture then being directly supplied to a mold cavity. Pressures as high as 1000 p.s.i. to 3000 p.s.i. are required to force the fluid streams through the small nozzles of the impingement mixer. An impingement mixing technique offers advantages in providing a low mixing chamber volume, easy cleaning thereof, and good mixing at high flow rates. However, such systems require that the fluids be delivered in the proper ratio by volume at all times which in turn requires that accurate control of the fluid flows is necessary if the most effective use of impingement mixing chambers is to be made. In most applications the volume ratio must be maintained to within 1% accuracy and in certain applications accuracies of 0.1% or better are required.